In general, in order to maintain high capacity operation of an alternating current servo motor, either the voltage or the current of the stator of an alternating current motor should be controlled. In the case of driving an alternating current servo motor, the position of the rotor is detected by using a position sensor such as a resolver or an encoder, and a stator current instruction value should be generated by utilizing said detected positional information so that the magnetic flux of the stator generated by three (3) phase stator current and the magnetic flux of the permanent magnet rotor are mutually operated whereby maximum torque is generated. On the other hand, since the stator coil winding of an alternating current servo motor includes a resistance element and an inductance element, current practically flowing to the stator coil winding comes to have a phase delay represented by a function of rotor speed as shown by the following expression (I) with respect to the current instruction value. And, since the bandwidth of the current control loop is limited by the magnitude of direct current supplied to inverter consisting of a power transistor, IGBT and MOSFET, the current flowing to the stator coil winding comes to have a phase delay which varies in response to the rotor speed with respect to the direct current instruction value. Therefore, the stator coil winding current of an alternating current servo motor Iu, Iv, Iw are given by the following formula (I), ##EQU1## Wherein, Vm represents a maximum value of stator phase voltage, L.phi. is self-inductance of each of the stator coil windings, .phi.m is the magnetomotive force of the permanent magnet, M is the mutual inductance between stator coil windings, Wr is the angular velocity of the rotor, .THETA.r is the angular displacement amount of the motor, and Rs is the resistance value of the stator coil winding.
A conventional control device of an induction motor is illustrated in Japanese patent official publication No. Sho-62-55398. However, in such a control device of an induction motor the reference value of the primary current for feeding to the induction motor is computed as an instantaneous value including a slipping on the basis of both commanding values, a torque commanding value and a magnetic flux commanding value, a mutual inductance value between the primary coil winding and the secondary coil winding of the induction motor and a magnetic flux commanding value and each set value of the induction motor, so that a frequency converting device for feeding primary current to the induction motor on the basis of said reference value is controlled. The control device includes means for obtaining a secondary magnetic flux from a respective output value of the primary voltage and the primary current of the induction motor, means for obtaining a secondary magnetic flux from the torque commanding value and the magnetic commanding value, and means for correcting the set value of the secondary coil winding resistance so that both secondary magnetic fluxes are equal, and then rendering the set value of the secondary coil winding resistance to follow its real value. This conventional device provides advantages in that torque corresponding linearly to torque command can be generated, magnetic flux response to the magnetic flux command becomes correct and speed control characteristics become better, however, there are also the disadvantages that this structure is relatively complicated and manufacturing cost is high.
As another example of a conventional apparatus, there is an analog motor current phase delay compensating apparatus shown in FIG. 1.
As shown in FIG. 1, this conventional current phase delay compensating apparatus is comprised of: a multiplier 1 for multiplying a proportional integral (PI) control output signal of a speed control gear with a resolver output signal, neither of which are shown; a phase compensating amount generator 2 for generating a phase amount to compensate by receiving an angular velocity of the rotor of the motor which is not shown; synchronous detection controller 3 for executing synchronous detection by receiving a output signal of said phase compensating amount generator 2; a circuit 4 for synchronously detecting by receiving an output signal from said multiplier 1 and synchronous detection controller 3; and a low bandpass filter 5 which receives an output signal of said synchronous detecting circuitry 4 and outputs U-phase and V-phase current instructions.
However, the conventional analog phase compensating apparatus of FIG. 1 has the drawbacks that not only is the configuration of the circuit complicated but also the phase delaying amount according to the stator coil winding current cannot sufficiently be compensated. Further, in the case of controlling various types of alternating current servo motors by means of one type of driving means, correction of the circuit is required in order to adjust the compensating amount for every respective type of alternating current servo motor.